The term tracer has generally been used to denote a material which is deliberately introduced into fluid flow which is taking place. Detection of the tracer(s) downstream of the injection point(s) provides information about the reservoir or about the wellbore penetrating the reservoir. In particular, deliberate addition of tracers has been used to observe flow paths and transit times between injection wells (used for instance to inject a water flood into a reservoir) and production wells. For this application of tracers to study inter-well flow, the tracer materials have generally been dissolved in the injection water at the surface before it is pumped down the injection well.
Some prior documents have proposed placing tracers in a well, or adjacent to it in a perforation extending through well casing into the surrounding formation, so as to monitor flow or events within the well rather than flow between wells. U.S. Pat. No. 507,771 proposed injecting radioactive tracers into perforations and monitoring loss of tracer with a wireline tool. U.S. Pat. No. 5,892,147 and U.S. Pat. No. 6,645,769 both proposed releasing distinguishable tracers from various underground locations within a wellbore and monitoring the produced flow to detect the presence of tracer. U.S. Pat. No. 6,840,316 proposed that tracer should be released under electrical control at various points within a complex hydrocarbon well and suggests a number of possible positions for sensors (of unspecified construction) to detect the presence of tracer.
In these various applications a deliberately added tracer may be present at very low concentration in the produced fluid where it is detected, and a number of prior documents have been concerned with choice of tracer material and methods of detection such that the tracer is detectable at very low concentrations. Substances deliberately introduced as tracers have included radioisotopes, fluorine-containing compounds and compounds of rare earth elements.
Whilst there are a variety of tracers and a variety of detection methods, a number of methods for detection of tracers involve the use of laboratory instruments. For example Society of Petroleum Engineers paper SPE 124689 proposes laser spectroscopy as a method of detection. WO2007/102023 proposes the use of a tracer containing a rare metal (e.g. caesium, hafnium, silver and gold) which is then detected in a sample by means of inductively coupled plasma mass spectrometry (ICP-MS).
When tracers are used, especially when the tracers do not contain radioactive isotopes, it is normal that samples are taken from the produced flow and sent away to a laboratory where solvent extraction or some other preparative procedure is carried out manually to extract and/or concentrate the tracer, after which the amount of tracer is determined by an analytical method which may be a sensitive instrumental technique. In consequence there is apt to be a significant time delay between taking the sample and obtaining an analysis of tracer(s) within it.